Skyrocketting costs and labour shortages are making it more and more urgent that a method of hard-rock mining alternative to the present drilling and blasting procedure be evolved. By "hard-rock" mining is meant the mining of rock such as is encountered in gold mining on the Witwatersrand in the Republic of South Africa and the gabbro complex at Duluth in the State of Minnesota, both of which have a hardness of the order of 7 on the Moh's scale. The most promising alternative appears to be rock-cutting or slotting, in which the reef is isolated from the waste rock, as much as is possible, by slots cut into the rock face, which cause or allow the reef and the waste to be separately detached and removed. There may be two slots, straddling the reef, producing such instability in the waste rock above the reef that the rock disintegrates or can be made to disintegrate, leaving the reef, itself made unstable by the slotting, to be separately detached and recovered for processing. In other cases, a single slot may suffice.
Techniques of this kind are known and have been successfully employed for mining soft materials such as coal, which has a hardness of the order of 3 on the Moh's scale. But mining hard rock of this nature poses massive problems not encountered in the mining of soft material such as coal so that technology evolved for mining such soft material successfully is so little helpful when dealing with the mining of hard rock as, in effect, to constitute a different art.
Two cutting methods of slotting hard rock are currently under development: one in which the slot is made by scraping a blade along the face and progressively deepening the slot at each traverse, until the maximum practical depth has been achieved. The other, and it is to this category that the present invention belongs, makes the slot by means of a percussive drill that is caused to traverse the face and progressively increase the depth of slot. This latter method has the attraction that the maximum depth of the slot is not dictated by the strength of the structure supporting the cutting elements, as in the former case, but by the length of the drill tool.
Two methods of percussive slot cutting have been proposed, each using a conventional rock drilling machine mounted to be traversed along the face. In one, the movement of the machine is rectilinear, in the other the path is arcuate. The present invention is applicable to both.
In the known slotting equipment, the rock drill is so mounted on its support that the drill tool is perpendicular to the general plane of the face. This arrangement imports disadvantages, which will be considered later, and which it is the purpose of the invention to minimise.
There is also known a rock slotting equipment disclosed in U.S. Pat. No. 2,398,311 to George W. Hulshizer, in which a percussive rock drill traverses the face of the material to be cut on an arcuate oscillating path, the drill axis being inclined to the tangent of the slot being cut at an angle, in the direction of advance of the drill, of about 105.degree.. The cutting operation proceeds on both the forward and the return strokes of the drill which is feasible owing to the fact that the machine is intended for operation in coal and like deposits, which are soft. The machine could not be operated in hard rock because, on the return stroke, the drill would not, owing to its acute angle to the tangent of the slot in the direction of movement of the drill, be capable of making a cut and would, in all probability, be damaged, as it juddered back to its initial position, and could also jam in the slot.